Control units for automotive engines are disposed in a confined space in an environment subject to high ambient temperatures and extreme vibrations. Typically, in order to enhance thermal performance and protection against engine vibrations, a printed circuit board that includes control circuitry is affixed to a rigidizer that may be bent to produce a reduced size module and to also dissipate heat. For example, in an application using a printed circuit board that has a bent flexible circuit, as is used for packaging an electronic control unit, the printed circuit board is mounted to an aluminum rigidizer. The rigidizer provides mechanical support for the printed circuit board and assists in the dissipation of heat generated by components on the printed circuit board by conductively transferring the heat from the components to the underlying rigidizer.
Referring now to FIGS. 1 and 2, an electronic control unit (ECU) 100 of the prior art is illustrated. FIG. 1 is an isometric perspective of an end view of electronic control unit 100. FIG. 2 is a cross-sectional side view of electronic control unit 100. Electronic control unit 100 includes a flexible printed circuit board 104 that is affixed to a rigidizer 106 by use of an adhesive 202. Typically, circuit board 104 has a number of electronic components 102 mounted on the surface thereof. Adhesive 202 is disposed between rigidizer 106 and printed circuit board 104 and functions to secure the printed circuit board to the rigidizer. After being secured to the flat rigidizer, the rigidizer and printed circuit board assembly is folded, resulting in a folded printed circuit board consisting of two main portions 204, 208 at an angle to each other and connected by a bent section 206 captured within a recess 112 of the bendable region 116 of the rigidizer 106. To provide a bend in the circuit board, a flexible circuit board is used. Unfortunately, flexible circuit boards cost more than their rigid counterparts. For example, a flexible fiberglass-weave epoxy (FR4) circuit board costs more than a rigid FR4 circuit board. Basically, the price differential is due to the problems that manufacturers have in handling the flexible boards through panel plating and solder mask operations, resulting in lower yields and subsequently higher costs.
In addition, flexible circuit boards have become more complex. As the functionality of electronic control units has increased over time, the corresponding circuitry has become increasingly dense and complex. As a result, electronic control units have been migrating from the use of two-layer printed circuit boards, such as printed circuit board 104, to the use of four-layer printed circuit boards. One result of the manufacturing process of four-layer printed circuit boards is a thicker flexible circuit, even when the bend section remains a two-layer circuit. Due to their thickness, flexible circuit boards are now known to crack and split when bent, unless a large bend radius is achieved, resulting a control module that must be discarded. The discarded control modules result in excessive manufacturing costs and waste, especially since the printed circuit boards must be populated with components before being folded.
One solution has been to provide fewer layers in the circuit board at the bend section than in the flat sections. Another solution has been to heat the circuit board during bending. Both of these solutions require additional unusual processing steps in either the manufacturing of the circuit board or the assembly of the electronic control unit.
Therefore, a need exists for a method and apparatus to provide the benefits of rigid circuit boards in a flexible printed circuit board assembly.